A Galaxy at Redshift 11.1?

Back in the office after one Friday off and there’s the inevitable queue at my door and mountain of things that just have to be done immediately. Yeah, right..

Anyway, I couldn’t resit a short blogging break to mention a bit of news that made a splash last week. This is the claim that a galaxy has been observed at a redshift z=11.1 which, if true, would make it the most distant such object ever observed. When I was a lad, z=0.5 was considered high redshift!

If the current standard cosmological model is correct then the lookback time to this redshift is about 13.4 billion years, which means that the galaxy we are seeing formed just 400 million years after the Big Bang. If it is correctly identified then it has to be an object which is forming stars at a prodigious rate. You can find more details in the discovery paper (by Oesch et al.) here.

I have taken the liberty of extracting the following figure:

The claim is that the model spectrum on the top right is a much better fit to the data obtained using the Hubble Space Telescope Grism spectrograph than the two alternatives at much lower redshift. However, this depends a great deal on having a good model for the significant contamination from other sources. Moreover I’m sure the residuals are non-Gaussian and I’m not therefore convinced that a simple χ2 is the best way to assess the fit. Obviously I’d like to see a proper Bayesian model comparison!

So, as I have been on previous occasions (e.g. here), I remain not entirely convinced. But then I’m a theorist who is always excessively suspicious of data. Any experts out there want to tell me I’m wrong?

7 Responses to “A Galaxy at Redshift 11.1?”

We just had a journal club on this paper. Steve Warren, who has form for eliminating claimed high z galaxies, was sufficiently convinced that he’d give 2:1 odds that this was a real z=11.1 source, but he wouldn’t offer better than that. Make of that what you will.

A joke, of course (note the smiley). However, expansion does change the redshift in that the redshift of an object which is stationary in co-moving coordinates will, in general, change with time. I’m not sure who first suggested this explicitly, but certainly Sandage did, a long time ago. Avi Loeb picked up on it much later, pointing out the spectroscopes had become sensitive enough to look for this effect during, say, an academic career. There are now even plans to do this at ESO.

I thought most of the analysis in the paper was good, but that the gloss the authors tried to put on it was not so good. They have done a great job to try and get the most information they can from the HST prism spectroscopy. The spectrum provides pretty good evidence that the colours cannot be explained by a z~2 galaxy with strong emission lines. This was the most likely low-redshift alternative. So the spectrum certainly increases the probability this source is at high redshift z=11, beyond what was already known from the photometry. But the gloss they put on it is that they have now secured a ‘spectroscopic redshift’. This is the gold standard, that says you know the distance. It used to mean a redshift confirmed by two lines (emission or absorption), but a single line that has the asymmetric shape of Lya at high z, detected at high S/N, is also convincing. I don’t think a break in the continuum measured at very low S/N matches the standard required to label this a ‘spectroscopic redshift’. My personal measure of a spectroscopic redshift is one that I would be happy to display in an outreach talk, and feel confident that I could convince the audience that it is correct. This one doesn’t meet that measure by some margin!